To contribute, first clone or fork this project. Once you've done that, create a new branch on your local machine to work on a feature.
git branch <your-branch-name>
Make sure your branch name is descriptive and clearly indicates what the branch is all about. If you'd like to work on multiple features, use multiple branches.
When your work is complete, push your branch to the remote repo.
git push origin <your-branch-name>
Once you've done that, you can go to github and create a new pull request. Someone will look over your code and there will be some discussion before a full-fledged merge.
It is really important that we try to do this project TDD-style as much as possible. This makes for more modular and maintainable code. Below are some useful links for testing.